The present invention relates to a printing head for an impact matrix printer.
The impact matrix printer comprises a plurality of printing elements, each comprising a permanent magnet, a leaf spring having a stylus, and an electromagnet consisting of a core and a coil. When the electromagnet is energized, the leaf spring is released, so that the stylus impacts the surface on which printing is performed.
In such a printer, a thin plate or film of nonmagnetic material such as stainless steel or polyimide resin is disposed between the top of the core and the leaf spring in order to enhance the release of the leaf spring from the top of the core upon the energization of the electromagnet, and to absorb the shock at the impact of the leaf spring on the top of the core. However, the film of polyimide resin becomes soft at a high temperature of over 100.degree. C., so that the film becomes broken by impact of the leaf spring.
In a printer in which the thin film of stainless steel is cantilevered, the leaf spring is attracted to the top of the core by the permanent magnet, interposing the film. When the coil is energized, the leaf spring rotates about a pivot and also the film rotates following the leaf spring about a center adjacent the cantilevered portion. In such a printing mechanism, the film slightly moves and slides on the top of the core in the radial direction, when disengaging and impacting from and on the top of the core. The movement of the film in the radial direction is caused by the rotational movement of the film. The sliding of the film on the top of the core causes the core and the film to wear. Since the core is made of a soft metal, it may become considerably worn.
Accordingly, the air gap between the film and the top of the core changes with the wearing, which results in deterioration of the printing quality. In addition, if the thin film is secured to a support by the welding thereof, the film may be corrugated. Accordingly, the amount of the air gap increases, resulting in an increase of differences among individuals in air gap, thereby causing extreme decreases of impact forces at particular printing elements.